This application claims the priority of Korean Patent Application No. 2003-68325, filed in the Korean Intellectual Property Office on Oct. 1, 2003, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention may relate to a liquid fuel mixer and a direct liquid feed fuel cell having the same. More particularly the invention may relate to a liquid fuel mixer having a permeable membrane that allows liquid fuel to permeate into water or a dilute liquid fuel through micropores, and to a direct liquid feed fuel cell having the same.
2. Description of Related Art
A direct liquid feed fuel cell may be an apparatus that generates electricity by electrochemical reaction of an organic fuel, such as methanol or ethanol, and an oxidant, such as oxygen. Since fuel may be directly fed to the cell, the direct liquid feed fuel cell may have several advantages such as very high energy density and power density. It may also have the advantages of not requiring a peripheral device such as a reformer, and of easing fuel storing and supply.
As depicted in FIG. 1, the direct liquid feed fuel cell may include an anode 2, a cathode 3, and an electrolyte membrane 1 interposed between the two electrodes 2 and 3. The anode 2 and cathode 3 may include diffusion layers 22 and 32 for supplying and diffusing fuel, as well as catalyst layers 21 and 31 for oxidation and reduction reaction of the fuel and oxygen. The anode 2 and cathode 3 may also include and electrode support layers 23 and 33 respectively. The catalyst for electrode reaction may include a precious metal having a superior catalytic characteristic at low temperature such as platinum. However, in order to avoid catalyst poisoning caused by a by-product from the reaction, e.g., CO, it may be desirable to select an alloyed catalyst containing a transition metal such as ruthenium, rhodium, osmium, or nickel. Waterproofed carbon paper or carbon cloth for easy fuel supply and dissipation of the reaction products may be used for the electrode support. An electrolyte membrane may be a polymer membrane having a thickness in a range of approximately 50˜200 μm.
A direct methanol fuel cell (DMFC) may be an example of a direct liquid feed fuel cell. A proton exchange membrane may be used as the electrolyte membrane. The electrochemical reaction of the DMFC may include an anode reaction in which fuel may be oxidized and a cathode reaction in which an oxidant may be reduced.
An example of each reaction can be described as follows.
[Reaction 1]CH3OH+H2O→CO2+6H++6e−  (Anode reaction)[Reaction 2] 3/2 O2+6H++6e−→3H2O  (Cathode reaction)[Reaction 3]CH3OH+ 3/2 O2→2 H2O+CO2   (Overall reaction)
At the anode 2 where an oxidation reaction (reaction 1) occurs, one carbon dioxide, six hydrogen ions, and six electrons are produced. The produced hydrogen ions migrate to the cathode 3 through a proton exchange membrane 1. At the cathode 3 where a reduction reaction (reaction 2) takes place, water may be produced by the reduction reaction between hydrogen ions, electrons transferred from an external circuit, and oxygen. Accordingly, water and carbon dioxide may be produced as a result of an electrochemical reaction (reaction 3) between methanol and oxygen.
A theoretical voltage output from a single cell of a DMFC may be approximately 1.2 V. However, an open circuit voltage at ambient temperature and atmospheric pressure falls below 1 V due to a voltage drop caused by an activation overpotential and a resistance overpotential. Thus, under practical conditions, the operating voltage may lie in the approximate range of 0.4˜0.6 V. Accordingly a plurality of single cells connected in series may be required to obtain higher voltages.
A stack cell may be formed by stacking several single cells connected in series electrically. Adjacent single cells electrically connected to each other by an electrical conductive bipolar plate (not shown) may be interposed between the single cells.
The bipolar plate (not shown) can be formed of a graphite block that may have high mechanical strength, high electrical conductivity, and good machining properties. A block of a composite material containing a metal or a polymer can be also used as the bipolar plate.
Flow channels for independently supplying fuel and air may be formed on the both faces of the bipolar plate. The bipolar plate placed within the stack may have a channel for supplying fuel on a face and a channel for supplying air on an opposite face, and bipolar plates (more precisely, monopolar plates because only one face may be working) placed on the uppermost or the lowermost of the stack may have a channel for supplying fuel or air.
Generally, flow channel 91 for supplying air or fuel may be formed on an entire surface of a conductive plate 9 in series and parallel or in a serpentine shape (other shapes are not excluded) so as to flow air or fuel. As illustrated in FIG. 2, the flow channel 91 may have a serpentine shape. Such a device may include an inlet 91a for fuel or air, and an outlet 91b for fuel or air.
A fuel supply system for supplying fuel to the fuel cell or fuel cell stack comprises a fuel tank for storing liquid fuel, a fuel pump for transferring liquid fuel from the fuel tank to the fuel cell or fuel cell stack, and a compressor or an air pump for supplying oxidant, e.g., air.
Since methanol and water theoretically reacts 1:1 (mole ratio) according to the above equation (reaction 1), it may be possible to use a mixture (approximately 64% by weight) of 1 mole of methanol to 1 mole of water. However, if the methanol concentration is too high, such as methanol to water ratio may be 1:1, then methanol crossover through the electrolyte membrane, thereby reducing the efficiency of the fuel cell. Therefore, in general, a low concentration of 2˜5M (6˜16% by weight) of methanol may be used. On the other hand, if the methanol concentration is too low, which means that a ratio of methanol may be very low in a given volume, energy output, for example, electricity generation may be very small. In order to get a higher energy output, it may be necessary to supply a large amount of fuel by a fuel pump to a stack cell.
FIG. 3 is a schematic illustration of a typical mixed fuel circulation system of a DMFC showing supply and recovery of a fuel mixture. As shown in FIG. 3, air for reduction reaction may be supplied to a cathode of a fuel cell stack 4, and the unreacted air at the cathode may be exhausted to atmosphere. A liquid fuel, e.g., a mixture of methanol and water from a mixed fuel tank 5, may be pumped to an anode in the fuel cell stack 4 and an unreacted fuel may be circulated to the mixed fuel tank 5 by a fuel pump 6.
In such a fuel circulating system, consumption of methanol in liquid fuel by the electrochemical reaction dilutes the methanol concentration of the liquid fuel in the fuel cell stack and the mixed fuel tank. Consequently, the increase in the water content and the decrease in the methanol content in the mixed liquid fuel cause gradual degradation of power generation efficiency. Also, a DMFC system using a diluted liquid fuel as depicted in FIG. 3 may have a drawback in that operation for long hours may be difficult because the storing capacity of mixed fuel may be limited by fuel storage and the methanol concentration in the mixed fuel tank becomes diluted as the operation proceeds.
As a solution to this problem, a method wherein mixed fuel may be supplied to the fuel cell stack after being mixed in a fuel mixer by taking methanol and water from separated storages was proposed and disclosed in U.S. Pat. No. 6,303,244.
FIG. 4 is a schematic illustration of another mixed fuel circuit of a DMFC system that may have a methanol storage separate from a water storage.
As shown in FIG. 4, air for a reduction reaction may be supplied to a cathode into the fuel cell stack 4, and the unreacted air at the cathode may be exhausted to the atmosphere. The water that may be produced as a by-product of the electrochemical reaction may be recovered to a water storage 6. Methanol in a high concentration or pure methanol may be stored in a methanol storage 7.
Liquid fuels, e.g., methanol and water, may be stored in separate storages 6 and 7, respectively, and pumped by an individual pump P to a fuel mixer 8 from the separate storages 6 and 7, and then the mixed fuel may be supplied to an anode of the fuel stack 4.
This system may have advantages in that the volume of water storage may be largely reduced because methanol and water may be stored separately, and long term operation may be possible since the storing volume of methanol can be increased. However, this system also may have the disadvantage of requiring an additional fuel mixer for mixing the separately stored methanol and water. Also, this system may have another disadvantage in that since a mixed fuel may be recycled by supplying and recovering, the separation of unreacted methanol from water may be practically impossible. The mixed fuel should be circulated in the mixed state as a system in FIG. 3, thus there may be a same drawback from the circulation of mixed fuel.
Meanwhile, a system of controlling a ratio of water to methanol by measuring the methanol concentration at the fuel mixer using a methanol sensor was disclosed in U.S. Pat. No. 6,306,285. Nevertheless, although a medium or large fuel cell system that generates more than hundreds of Watt can use the methanol sensor, a small fuel cell system can hardly adopt the methanol sensor because of increase in the weight and volume of system.